1. Field of the Invention
The present invention relates to electrical machines having a stator and a rotor. More particular, the present invention relates to permanent magnet electrical machines wherein the permanent magnet is locked to the rotor of the machine. Additionally, the present invention relates to apparatus for locking the permanent magnet to an outer surface of a rotor slice of the electrical machine.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Permanent magnets make it possible to build electrical machines (otherwise known as “motors” and “generators”) using magnets for field generation other than electro-magnets. Typically, the magnets are mounted on an inner rotor. However, outer rotor mounting can also be used. In a generator, the rotation of the magnetic field of the magnets on the rotor induces voltages and currents in the radially outward stator. In a motor, the magnets react to voltages and currents that are applied to the stator and cause rotation to the rotor.
Permanent magnet electrical machines are more compact and simpler. They require less maintenance than their electromagnetic counterparts by not requiring electromagnetic windings, along, with their associated connections, brushes and exciters. Modern rare earth magnets provide a dense source of magnetic flux than can windings. Such magnets also have higher flux and are able to withstand reasonably high temperatures. The resulting compact machines find application in structures where size, weight and efficiency are important. One application for such permanent magnet electrical machines is within the nacelles of wind turbines located at the top of high towers. Additionally, such permanent magnet electrical machines can be used as motors where space is at a premium.
One of the manufacturing issues related to permanent magnet machines is the mounting of the magnets onto the rotor. Magnets, especially those made from magnetic rare earth material, are heavy, brittle and hard. As such, if a magnet where to break free from the rotor, the stator and the rotor would be damaged by the impact of the debris.
Conventionally, magnets are mounted by bonding them onto the rotor surface. Although the adhesive is typically an epoxy, this alone is often inadequate in view of the powerful centrifugal forces on the magnets due to the rotor rotation. Additionally, such adhesives can also be inadequate in view of the magnetic attraction of the magnets to the ferromagnetic material or induced field of the stator, which is separated from the rotor magnets by only a small air gap.
Banding with a non-ferrous taped material (such as fiberglass, carbon fiber or KEVLAR™ tape) is used in a manner in which the tape is tightly wound around the circumference of the bonded magnets. Such magnets are also adhesively bonded to the rotor. This approach adds the thickness of the tape to the air gap distance between the magnets and the stator. This could cause a reduction of magnetic flux from the rotor to the stator and can also cause a blockage of cooling air in this gap.
Another conventional manner that is used to attach magnets is to embed them in the rotor structure so that they are held tightly within the ferrous structure of the rotor. This approach adds the embedded thickness to the distance between the magnets and the stator so as to cause a reduction of magnetic flux to the stator. It also causes a partial flux short-circuit within the rotor, since the embedding material is magnetic, so as to result in a further reduction of magnetic flux to the stator.
A further approach for attaching a magnet includes the use of clamps that are captured in cavities within the rotor. This approach requires precision components. The clamps and the cavities can also add to the cost and reliability of the machine.
Electric motors are characterized as “slotted” in which the rotor is segmented axially with coolant slots between the segments, “continuous” in which the rotor is not slotted, “solid-metal” in which the rotor or rotors segments are formed from solid ferromagnetic material, and “laminated” in which the rotor or rotor segments are formed from ferromagnetic lamination to minimize eddy current losses and the laminations are stacked to form the rotor or rotor segment.
FIG. 1 illustrates a conventional surface-mounted machine 10 where the magnets 12 are simply adhered to the outer surface 14 of the rotor core 16. Each of the magnets 12 are separated from each other using non-ferromagnetic spacers 18 between proximal magnets 12. The non-ferromagnetic spacers 18 are typically formed of a fiberglass material. A shaft can be provided for the rotation of the core 16. The approach shown in FIG. 1, by placing the magnet 12 closest to the surrounding stator across the rotor-stator gap, would be ideal if the adhesive were strong enough to counter the centrifugal force of rotation and any magnetic attraction to the stator. Generally, the simple approach does not sufficiently hold the magnets.
FIG. 2 illustrates a conventional banded machine 20 where the magnets 22 are adhered to the outer surface 24 of the rotor core 26. The magnets 22 are separated by non-ferromagnetic spacers 28 between the proximal magnets 22. The bands 30 are cinched around the magnets 22 to hold them in place. Although this approach actually worked, it places the band or bands 30 between the magnets 22 and the stator 32. As such, the band 30 will reside in the rotor-stator gap 34. As a result, in order to accommodate the bands, it may be necessary to increase the size of the gap 34. The magnetic flux efficiency would be correspondingly reduced.
FIG. 3 illustrates a conventional machine 40 that uses non-ferromagnetic clamps 44 captured in the rotor core 46. The clamps 44 overlap the magnets 42 to hold the magnets 42 to the surface 48 of the rotor core 46. Cavities 50 are provided on the rotor core 46 to capture the clamps 44. If the clamps 44 do not extend too far above the magnets 42 into the rotor-stator gap 52, the clamps 44 would not interfere with the flux generator. However, the formation of the cavities 50 within the rotor core 46 requires precise laminations for laminated rotors, or substantial milling to carve them out of solid-metal core rotors. The clamps 44 are also precision components. The insertion of these clamps 44 into the cavities 50 requires a great deal of precision during assembly.
FIG. 3 further illustrates the use of spider spokes 54 in order to attach the rotor core 46 to the shaft 56 while reducing the amount of material used for the rotor core 46. The resulting space 58 between the shaft 56 and the rotor core 46 is generally used as a passageway for machine coolant. The spider spokes 54 are welded onto or into the laminations of a laminated rotor machine. The spider spokes 54 can also be welded onto or cast as part of a solid-metal core material.
It is an object of the present invention to provide a rotor assembly whereby the permanent magnets can be attached to rotor slices so as to not interfere with the effective magnetic flux from the magnet of said stator.
It is another object of the present invention to provide a rotor assembly whereby the permanent magnets are securely mounted to a circumferential surface of the rotor slices.
It is still a further object of the present invention to provide rotor assembly whereby the permanent magnets can be attached in a simple and easy-to-assembly manner.
It is a further object of the present invention to provide at a rotor assembly which requires no machining of the rotor core or the permanent magnets.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.